Photovoltaic cell comprising a thin lamina having a rear junction and method of making

ABSTRACT

Fabrication of a photovoltaic cell comprising a thin semiconductor lamina may require additional processing after the semiconductor lamina is bonded to a receiver. To minimize high-temperature steps after bonding, the p−n junction is formed at the back of the cell, at the bonded surface. In some embodiments, the front surface of the semiconductor lamina is not doped or is locally doped using low-temperature methods. The base resistivity of the photovoltaic cell may be reduced, allowing a front surface field to be reduced or omitted.

RELATED APPLICATIONS

This application is related to Herner et al., U.S. patent applicationSer. No. 12/189,156, “Method to Mitigate Shunt Formation in aPhotovoltaic Cell Comprising a Thin Lamina,” and to Herner et al., U.S.patent application Ser. No. 12/189,159, “A Photovoltaic ModuleComprising Thin Laminae Configured to Mitigate Efficiency Loss Due toShunt Formation,” both filed on even date herewith and owned by theassignee of the present application, and both hereby incorporated byreference.

This application is also related to Hilali et al., U.S. patentapplication Ser. No. 12/189,157, “Photovoltaic Cell Comprising a ThinLamina Having Low Base Resistivity and Method of Making,” filed on evendate herewith, owned by the assignee of the present application, andhereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method to form an efficient photovoltaic cellwhile minimizing fabrication costs.

For some fabrication techniques, it may be useful to minimize processingtemperature during later stages of fabrication. Many standard stepsperformed during fabrication of photovoltaic cells, such as diffusiondoping and oxidation, require high temperature.

There is a need, therefore, to form the structures of a photovoltaiccell while minimizing temperature.

SUMMARY OF THE PREFERRED EMBODIMENTS

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Ingeneral, the invention is directed to a photovoltaic cell comprising athin lamina having a p−n junction at the back of the cell. In someaspects of the invention, the resistivity of the base region of the cellmay be reduced below conventional levels.

A first aspect of the invention provides for a photovoltaic cellcomprising a semiconductor lamina, the semiconductor lamina having afront surface and a back surface, wherein incident light enters thesemiconductor lamina at the front surface during normal operation of thephotovoltaic cell, the body of the semiconductor lamina is doped to afirst conductivity type, the back surface of the semiconductor lamina isheavily doped to a second conductivity type opposite the firstconductivity type, and no more than ten percent of the surface area ofthe front surface of the semiconductor lamina is doped more heavily tothe first conductivity type than the lamina body.

An embodiment of the invention provides for a method for making aphotovoltaic cell, the method comprising: providing a donor wafer dopedto a first conductivity type; doping a first surface of the donor waferto a second conductivity type opposite the first conductivity type;defining a cleave plane within the donor wafer; affixing the firstsurface of the donor wafer to a receiver element; cleaving asemiconductor lamina from the donor wafer at the cleave plane whereinthe semiconductor lamina remains affixed to the receiver element; andcompleting fabrication of the photovoltaic cell, wherein thephotovoltaic cell comprises the semiconductor lamina.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another.

The preferred aspects and embodiments will now be described withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art photovoltaic cell.

FIGS. 2 a-2 d are cross-sectional views illustrating stages in formationof an embodiment of Sivaram et al., U.S. patent application Ser. No.12/026,530.

FIGS. 3 a-3 e are cross-sectional views showing stages in formation of aphotovoltaic cell formed according to an embodiment of the presentinvention.

FIGS. 4 a and 4 b show an alternative embodiment of the presentinvention, the embodiment having dense contacts at its front surface.FIG. 4 a is a plan view, while FIG. 4 b shows a cross-sectional view.

FIG. 5 is a cross-sectional view of another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A conventional prior art photovoltaic cell includes a p−n diode; anexample is shown in FIG. 1. A depletion zone forms at the p−n junction,creating an electric field. Incident photons will knock electrons fromthe conduction band to the valence band, creating free electron-holepairs. Within the electric field at the p−n junction, electrons tend tomigrate toward the n region of the diode, while holes migrate toward thep region, resulting in current. This current can be called thephotocurrent. Typically the dopant concentration of one region will behigher than that of the other, so the junction is either a p+/n−junction (as shown in FIG. 1) or a p−/n+ junction. The more lightlydoped region is known as the base of the photovoltaic cell, while themore heavily doped region is known as the emitter. Most carriers aregenerated within the base, and it is typically the thickest portion ofthe cell. The base and emitter together form the active region of thecell.

Conventional photovoltaic cells are formed from monocrystalline,polycrystalline, or multicrystalline silicon. A monocrystalline siliconwafer, of course, is formed of a single silicon crystal, while the termmulticrystalline typically refers to semiconductor material havingcrystals that are on the order of a millimeter in size. Polycrystallinesemiconductor material has smaller grains, on the order of a thousandangstroms. Monocrystalline, multicrystalline, and polycrystallinematerial is typically entirely or almost entirely crystalline, with noor almost no amorphous matrix. For example, non-deposited semiconductormaterial is at least 80 percent crystalline.

Photovoltaic cells fabricated from substantially crystalline materialare conventionally formed of wafers sliced from a silicon ingot. Currenttechnology does not allow wafers of less than about 150 microns thick tobe fabricated into cells economically, and at this thickness asubstantial amount of silicon is wasted in kerf, or cutting loss.Silicon solar cells need not be this thick to be effective orcommercially useful. A large portion of the cost of conventional solarcells is the cost of silicon feedstock, so decreasing the thickness of aphotovoltaic cell may reduce cost.

Sivaram et al., U.S. patent application Ser. No. 12/026,530, “Method toForm a Photovoltaic Cell Comprising a Thin Lamina,” filed Feb. 5, 2008,owned by the assignee of the present application and hereby incorporatedby reference, describes fabrication of a photovoltaic cell comprising athin semiconductor lamina formed of crystalline, non-depositedsemiconductor material. Referring to FIG. 2 a, in embodiments of Sivaramet al., a semiconductor donor wafer 20 is implanted with one or morespecies of gas ions, for example hydrogen or helium ions. The implantedions define a cleave plane 30 within the semiconductor donor wafer. Asshown in FIG. 2 b, donor wafer 20 is affixed at first surface 10 toreceiver 60. Referring to FIG. 2 c, an anneal causes lamina 40 to cleavefrom donor wafer 20 at cleave plane 30, creating second surface 62. Inembodiments of Sivaram et al., additional processing before and afterthe cleaving step forms a photovoltaic cell comprising semiconductorlamina 40, which is less than 100 microns thick, generally between about0.2 and about 100 microns thick, for example between about 0.2 and about50 microns, for example between about 1 and about 50 microns thick, insome embodiments between about 1 and about 10 microns thick, though anythickness within the named range is possible. FIG. 2 d shows thestructure inverted, with receiver 60 at the bottom, as during operationin some embodiments. Receiver 60 may be a discrete receiver elementhaving a maximum width no more than 50 percent greater than that ofdonor wafer 10, and preferably about the same width, as described inHemer, U.S. patent application Ser. No. 12/057,265, “Method to Form aPhotovoltaic Cell Comprising a Thin Lamina Bonded to a Discrete ReceiverElement,” filed on Mar. 27, 2008, owned by the assignee of the presentapplication and hereby incorporated by reference, hereinafter the '265application.

Using the methods of Sivaram et al., photovoltaic cells are formed ofthinner semiconductor laminae without wasting silicon through kerf lossor by formation of an unnecessarily thick wafer, thus reducing cost. Thesame donor wafer can be reused to form multiple laminae, furtherreducing cost, and may be resold after exfoliation of multiple laminaefor some other use. The cost of the hydrogen or helium implant may bekept low by methods described in Parrill et al., U.S. patent applicationSer. No. 12/122,108, “Ion Implanter for Photovoltaic Cell Fabrication,”owned by the assignee of the present application, filed May 16, 2008,and hereby incorporated by reference.

Conventional crystalline cells may be 200 or 300 microns thick. When thecell is exposed to sunlight, most free charge carriers are generated inthe first 10 microns of the light-facing surface of the cell. Inconventional crystalline cells formed from wafers, then, the p−njunction is generally formed at or near the front of the cell, so thatmost of the minority carriers generated in the base will have a shortdistance to travel to the emitter, where they are collected and thuscreate photocurrent. If the junction were formed at the rear of suchthick cells, minority carriers generated at the front of the cell wouldrecombine before reaching the emitter, and thus not generatephotocurrent. Such back-junction cells have been produced on thickersilicon wafers, but they require very pure, very expensive material withhigh lifetimes.

Along with the emitter region created at the front of the cell, aheavily doped region is also generally created at the back of the cell,to improve ohmic contact to the cell. Using conventional techniques,this cell structure is readily achieved: The wafer is doped as desiredon both sides, then affixed to a substrate or superstrate once allhigh-temperatures steps are complete.

Referring to FIG. 2 d, in many embodiments of Sivaram et al., heavilydoped regions are also formed both at first surface 10 and at secondsurface 62 of the photovoltaic cell, in order to define a p−n junctionand to provide ohmic contact to the cell. In embodiments of Sivaram etal., the donor wafer is affixed to receiver 60 before exfoliation sothat receiver 60 will provide mechanical support to thin lamina 40during and after exfoliation. Doping of second surface 62, which iscreated by exfoliation, thus typically takes place after bonding toreceiver 60. Formation of a heavily doped region at second surface 62generally requires a high-temperature step to introduce and activate thedopant.

Exposing lamina 40 to a high-temperature step while it is bonded toreceiver 60 entails the risk of damage to receiver 60, damage to thebonding itself, and of potential contamination to the semiconductorlamina by the bonding material. Careful selection of receiver andbonding materials will reduce or eliminate this risk.

In the present invention, this risk is avoided by forming the p−njunction at the back of the cell, at first surface 10 which is bonded toreceiver 60. Since lamina 40 is less than 100 microns thick, andtypically less than about 10 microns thick, for example two micronsthick or less, photogenerated minority carriers in the base do not havefar to travel to be collected at the emitter, even when this emitter isformed at the back of the cell. In fact, because any carriers generatedin a heavily doped region formed at the front of the cell will tend torecombine immediately, moving the junction to the back of the cell, intoa zone in which fewer carriers are generated, may provide an efficiencyadvantage. Doping at first surface 10 can be performed before bonding toreceiver 60.

With the p−n junction already formed, a variety of strategies may beemployed to avoid exposing lamina 40 and receiver element 60 to anyadditional high-temperature steps, such as diffusion doping. It ispreferred to avoid formation of a front surface field (FSF), which wouldrequire such a doping step. As will be described, a very fine, verytightly spaced grid can be formed contacting second surface 62 with noFSF at this surface. Alternatively, low-temperature methods may beemployed to form doped regions very locally, only where wiring contactsfront surface 62. In other embodiments, a cell is formed havingdecreased base resistivity, which will allow ohmic contact to be made tothe cell with no FSF and with more conventional spacing of grid lines.

Note this discussion describes embodiments in which second surface 62 isthe front, light-facing surface of the cell, while first surface 10 isthe back surface. Other arrangements are possible, as described inSivaram et al. and the '265 application. For example, receiver element60 may be transparent and serve as a superstrate. In this case firstsurface 10, which is bonded to receiver element 60, would be the frontsurface of the cell, while second surface 62 is the back surface.

For clarity, several examples of fabrication of a lamina havingthickness less than 100 microns, where the lamina comprises, or is aportion of, a photovoltaic cell according to embodiments of the presentinvention, will be provided. For completeness, many materials,conditions, and steps will be described. It will be understood, however,that many of these details can be modified, augmented, or omitted whilethe results fall within the scope of the invention. In theseembodiments, it is described to cleave a semiconductor lamina byimplanting gas ions and exfoliating the lamina. Other methods ofcleaving a lamina from a semiconductor wafer could also be employed inthese embodiments.

Example: Dense Grid Spacing, No or Localized FSF

The process begins with a donor body of an appropriate semiconductormaterial. An appropriate donor body may be a monocrystalline siliconwafer of any practical thickness, for example from about 300 to about1000 microns thick. In alternative embodiments, the wafer may bethicker; maximum thickness is limited only by practicalities of waferhandling. Alternatively, polycrystalline or multicrystalline silicon maybe used, as may microcrystalline silicon, or wafers or ingots of othersemiconductors materials, including germanium, silicon germanium, orIII-V or II-VI semiconductor compounds such as GaAs, InP, etc. In thiscontext the term multicrystalline typically refers to semiconductormaterial having grains that are on the order of a millimeter or largerin size, while polycrystalline semiconductor material has smallergrains, on the order of a thousand angstroms. The grains ofmicrocrystalline semiconductor material are very small, for example 100angstroms or so. Microcrystalline silicon, for example, may be fullycrystalline or may include these microcrystals in an amorphous matrix.Multicrystalline or polycrystalline semiconductors are understood to becompletely or substantially crystalline. The donor wafer is preferablyat least 80 percent crystalline, and in general will be entirelycrystalline.

The process of forming monocrystalline silicon generally results incircular wafers, but the donor body can have other shapes as well.Cylindrical monocrystalline ingots are often machined to an octagonalcross section prior to cutting wafers. Multicrystalline wafers are oftensquare. Square wafers have the advantage that, unlike circular orhexagonal wafers, they can be aligned edge-to-edge on a photovoltaicmodule with no unused gaps between them. The diameter or width of thewafer may be any standard or custom size. For simplicity this discussionwill describe the use of a monocrystalline silicon wafer as thesemiconductor donor body, but it will be understood that donor bodies ofother types and materials can be used.

Referring to FIG. 3 a, donor wafer 20 is formed of monocrystallinesilicon which is lightly to moderately doped to a first conductivitytype. The present example will describe a relatively lightly n-dopedwafer 20 but it will be understood that in this and other embodimentsthe dopant types can be reversed. Dopant concentration may be betweenabout 1×10¹⁴ and 3×10¹⁸ atoms/cm³; for example between about 2×10¹⁵ and7×10¹⁵ atoms/cm³; for example about 5×10¹⁵ atoms/cm³. Desirableresistivity for n-type silicon may be, for example, between about 44 andabout 0.005 ohm-cm, preferably about 2.5 to about 0.7 ohm-cm, forexample about 1.0 ohm-cm. For p-type silicon, desirable resistivity maybe between about 133 and about 0.01 ohm-cm, preferably between about 7and about 2 ohm-cm, for example about 2.8 ohm-cm.

First surface 10 is optionally treated to produce surface roughness, forexample, to produce a Lambertian surface. The ultimate thickness of thelamina limits the achievable roughness. In conventional silicon wafersfor photovoltaic cells, surface roughness, measured peak-to-valley, ison the order of a micron. In embodiments of the present invention, thethickness of the lamina may be between about 0.2 and about 100 microns.Preferred thicknesses include between about 1 and about 80 microns; forexample, between about 1 and about 20 microns or between about 2 andabout 20 microns. Practically, any thickness in the range between about0.2 and about 100 microns is achieveable; advantageous thicknesses maybe between about 1 and about 1.5, 2, 3, 5, 8, 10, 20, or 50 microns.

If the final thickness is about 2 microns, clearly surface roughnesscannot be on the order of microns. For all thicknesses, a lower limit ofsurface roughness would be about 500 angstroms. An upper limit would beabout a quarter of the film thickness. For a lamina 1 micron thick,surface roughness may be between about 600 angstroms and about 2500angstroms. For a lamina having a thickness of about 10 microns, surfaceroughness will be less than about 25000 angstroms, for example betweenabout 600 angstroms and 25000 angstroms. For a lamina having a thicknessof about 20 microns, surface roughness may be between about 600angstroms and 50000 angstroms.

Surface roughness may be random or may be periodic, as described in“Niggeman et al., “Trapping Light in Organic Plastic Solar Cells withIntegrated Diffraction Gratings,” Proceedings of the 17^(th) EuropeanPhotovoltaic Solar Energy Conference, Munich, Germany, 2001. Formationof surface roughness is described in further detail in Petti, U.S.patent application Ser. No. 12/130,241, “Asymmetric Surface TexturingFor Use in a Photovoltaic Cell and Method of Making,” filed May 30,2008, owned by the assignee of the present application and herebyincorporated by reference.

Next, first surface 10 is doped, for example by diffusion doping. Firstsurface 10 will be more heavily doped to the conductivity type oppositethat of original wafer 20. In this instance, donor wafer 20 is n-type,so first surface 10 is doped with a p-type dopant, forming heavily dopedp-type region 16. Doping may be performed with any conventional p-typedonor gas, for example B₂H₆ or BCl₃. Doping concentration may be, forexample, between about 1×10¹⁸ and 1×10²¹ atoms/cm³, for example about1.5×10²⁰ atoms/cm³.

Next, ions, preferably hydrogen or a combination of hydrogen and helium,are implanted to define a cleave plane 30, as described earlier. Notethat the plane of maximum distribution of implanted ions, and of implantdamage, is conformal. Any irregularities at first surface 10 will bereproduced in cleave plane 30. Thus, in some embodiments if firstsurface 10 is roughened, it may be preferred to roughen surface 10 afterthe implant step rather than before. Once the implant has beenperformed, exfoliation will occur once certain conditions, for exampleelevated temperature, are encountered. It is necessary, then, to keepprocessing temperature and duration below those which will initiateexfoliation until exfoliation is intended to take place.

Turning to FIG. 3 b, next conductive layer 12 is formed on first surface10. In most embodiments, layer 12 is also reflective. Silver may be anappropriate material. Other alternatives for such a layer, in this andother embodiments, include aluminum, titanium, chromium, molybdenum,tantalum, zirconium, vanadium, indium, cobalt, antimony, or tungsten, oralloys thereof. In some embodiments, it may be preferred to deposit athin layer 12 of silver onto first surface 10, though conductive layer12 may be formed by any appropriate method.

The surface of layer 12 is cleaned of foreign particles, then affixed toreceiver element 60. In alternative embodiments, layer 12 may have beenformed on receiver element 60, rather than on first surface 10 of donorwafer 20. Receiver element 60 is any appropriate material, for examplemetal, glass, plastic, etc. As shown in FIG. 3 c, lamina 40 can now becleaved from donor wafer 20 at cleave plane 30 as described earlier.FIG. 3 c shows the structure with receiver element 60 on the bottom, asduring normal operation once fabrication has been completed. Lamina 40remains affixed to receiver element 60 with conductive layer 12 disposedbetween them, as described in Herner et al., U.S. patent applicationSer. No. 12/057,274, “A Photovoltaic Assembly Including a ConductiveLayer Between a Semiconductor Lamina and a Receiver Element,” filed Mar.27, 2008, owned by the assignee of the present application, hereinafterthe '274 application and hereby incorporated by reference. Secondsurface 62 has been created by exfoliation. As has been described, somesurface roughness is desirable to increase light trapping within lamina40 and improve conversion efficiency of the photovoltaic cell. Theexfoliation process itself creates some surface roughness at secondsurface 62. In some embodiments, this roughness may alone be sufficient.In other embodiments, surface roughness of second surface 62 may bemodified or increased by some other known process, such as a wet or dryetch, or by the methods described by Petti, as may have been used toroughen first surface 10.

The implant step used to create the cleave plane will produce somedamage to the crystalline structure of the lamina. This damage will bemost extensive at the end of the range of implanted ions. This damagedarea will thus be at or near second surface 62. This damaged area couldcause excessive recombination of photogenerated carriers, as well asforming a layer that is difficult to electrically contact with a lowenough resistance. This damaged layer could be repaired byhigh-temperature annealing, but, as explained, it is preferred to avoidhigh-temperature processing at this point. Alternatively, the damagedlayer could be etched off, for example, in a solution of HF:H₂C₃O₂:HNO₃in a ratio of 2:5:15 for about 3-5 sec. Such an etching step wouldremove about 0.25 microns of silicon, which should contain all of thedamaged material for a typical cleaving implant.

A transparent dielectric layer 64 is formed on second surface 62. If, asin the present embodiment, the body of lamina 40 is lightly dopedn-type, then dielectric layer 64 is preferably silicon nitride, forexample formed by plasma-enhanced chemical vapor deposition. Siliconnitride also serves as an antireflective coating (ARC). Dielectric layer64 may be between about 500 and 2000 angstroms thick, for example about650 angstroms.

Next trenches 44 are opened in silicon nitride layer 64, for example bylaser ablation. Trenches 44 are between about 10 and about 50 micronswide, for example between about 10 and about 15 or 20 microns wide. Thepitch of trenches 44 is preferably between about 200 and about 1500microns, for example between about 200 and about 800 microns, forexample about 400 microns. The pitch and width of the lines will beadjusted to account for the material used to form wiring, the expectedcurrent from the cell, etc., as will be understood by those skilled inthe art.

Turning to FIG. 3 d, next a source of an n-type dopant such as aphosphorus dopant, for example phosphoric acid or phosphorus pentoxide,is formed on the regions of second surface 62 exposed in trenches 44,for example by screenprinting, aerosol printing, or inkjet printing. Bypassing a laser beam over the exposed regions of surface 62, theseregions 14 are locally heavily n-doped. Laser heating of lamina 40 willbe very local, typically only tens of nanometers deep, and is achievedwithout subjecting receiver element 60 or the body of lamina 40 to ahigh-temperature step. It will be understood that FIG. 3 d is not toscale, and heavily doped regions 14 are much thinner than as depicted.In general heavily doped regions 14 will be no more than ten percent ofthe surface area of second surface 62. It may be preferred for anyremaining undiffused dopant to be rinsed off with deionized waterfollowed by a quick buffered oxide etch to remove any phosphosilicateglass that may have formed, preferably followed by an additional rinsein dionized water.

Next wiring is formed in trenches 44 (shown in FIG. 3 c). Formation of avery thin nickel seed layer (not shown) on the exposed regions of secondsurface 62 is followed by electroplating of copper, or, for example,conventional or light-induced plating of either silver or copper. Theseplating methods selectively deposit the metal, forming wiring 57. Thethickness of wiring 57 will be selected to produce the desiredresistance, and may be, for example, about 7 to 10 microns.

In this embodiment, silicon nitride was selected for deposition onsecond surface 62 for several reasons. Silicon nitride generally isnaturally positively charged. In the cell described in this embodiment,when the cell is exposed to light, excess holes and electrons aregenerated throughout the cell, but especially near the light-facingsurface 62. If the interface at this surface contains a high density oftrapping sites, many or most of the photogenerated electrons and holeswill recombine at this surface and thus will create little or nophotocurrent. The positive charge of silicon nitride layer 64 tends torepel holes from this surface. Thus, even with a high density ofinterface traps, there will be few holes with which electrons canrecombine, allowing them to be more readily collected by wiring 57.Alternatively, in other embodiments, other ARCs may be used.

Next, turning to FIG. 3 e, photovoltaic assembly 82, which compriseslamina 40 and receiver element 60, is affixed to a substrate 90, or to asuperstrate, along with a plurality of other photovoltaic assemblies 82.Each photovoltaic assembly 82 comprises a photovoltaic cell. Thephotovoltaic cells are electrically connected in series, forming acompleted photovoltaic module.

Many variations are possible. In some embodiments, thin stripes of ann-type dopant can be formed on second surface 62, for example by inkjetprinting, before deposition of silicon nitride layer 64; in this casethe laser ablation step that opens trenches 44 can also serve to doperegions of second surface 62 and activate the dopant. In otherembodiments, deposition of a dopant source on the surface and the laserdoping step can be omitted, and no portion of second surface 62 will bedoped.

Referring to FIG. 4 a, which shows a portion of lamina 40 in plan view,in another alternative, laser ablation can be used to form holes 46rather than trenches in silicon nitride layer 64. Each hole may be, forexample, about 10 microns in diameter, and the pitch of the holes in onedimension can be, for example, about 20 microns. Referring to FIG. 4 b,which shows the same structure in cross-section along line A-A′, afteroptional laser doping in each hole, contact 56 is formed in each hole byformation of a nickel seed layer and silver or copper electrolessplating, as described earlier. Above the surface of silicon nitridelayer 64, contacts 56 will be larger than the 10 micron holes in whichthey are formed, and spacing can be selected so that adjacent contacts56 merge, forming a single gridline 59. Gridline spacing can be betweenabout 200 and about 1500 microns, for example between about 400 andabout 800 microns, for example about 500 microns. Note that drawings arenot to scale.

In this and other embodiments, polarity can be reversed. If the originaldonor wafer is p-type, first surface 10 will be doped heavily n-type,and a negatively charged dielectric such as alumina may be formed onsecond surface 62 to passivate the surface. Alumina is not an effectiveARC, so in this case the layer of alumina may be relatively thin, forexample between about 10 and about 30 nm, while an additional thickness,for example about 200 nm, of, for example, silicon oxide formed on thealumina serves as an ARC. Alternatively, the ARC may be about 70 nm oftitanium oxide, or a stack of about 10 nm of silicon oxide on thealumina, and about another 60 nm of silicon nitride on the siliconoxide.

Summarizing, what has been described is a photovoltaic cell comprising asemiconductor lamina, the semiconductor lamina having a front surfaceand a back surface, and wherein incident light enters the semiconductorlamina at the front surface during normal operation of the photovoltaiccell. In embodiments of this cell, the body of the semiconductor laminais lightly to moderately doped to a first conductivity type, the backsurface of the semiconductor lamina is heavily doped to a secondconductivity type opposite the first conductivity type, and no more thanten percent of the surface area of the front surface of thesemiconductor lamina is doped more heavily to the first conductivitytype than the lamina body. The front surface may comprise a plurality ofdiscrete more heavily doped regions of the first conductivity type, eachof which is in contact with conductive material such as wiring. In someinstances, none of the front surface is heavily doped. Generally withinabout 300 nm of the front surface of the semiconductor lamina, noportion of the semiconductor lamina has a dopant concentration greaterthan about 1×10¹⁹ atoms/cm³.

As described, the photovoltaic cell may be formed by providing a donorwafer doped to a first conductivity type; doping a first surface of thedonor wafer to a second conductivity type opposite the firstconductivity type; defining a cleave plane within the donor wafer;affixing the first surface of the donor wafer to a receiver element;cleaving a semiconductor lamina from the donor wafer at the cleave planewherein the semiconductor lamina remains affixed to the receiverelement; and completing fabrication of the photovoltaic cell, whereinthe photovoltaic cell comprises the semiconductor lamina. The cleavingstep may create a second surface of the semiconductor laminasubstantially parallel to the first surface, and fabrication of thephotovoltaic cell is completed by forming localized areas of dopant onthe second surface, and applying laser energy to the second surface toform heavily doped regions of the semiconductor lamina adjacent to thelocalized areas.

Example: Low Base Resistivity

In the prior example, no FSF or only a localized FSF was formed, and insome embodiments, the resistivity of the base region (the lightly dopedportion of the photovoltaic cell) was in a conventional range. In aconventional photovoltaic cell, base resistivity may be between about0.7 and about 2.5 ohm-cm. The inventors of the present invention havefound that in embodiments of the present invention, base resistivity canbe reduced significantly without degradation of conversion efficiency,and in some cases some improvement. When base resistivity is lower, itis expected that there will be less need for localized doping beneaththe electrical contacts of the front surface to reduce contactresistance, thus simplifying fabrication.

In the prior example, to compensate for the lack of an FSF, grid spacingwas tighter than in conventional photovoltaic cells. When baseresistivity is reduced, for example to less than about 0.5 ohm-cm, forexample between about 0.005 ohm-cm and about 0.1 ohm-cm, for examplebetween about 0.01 ohm-cm and 0.05 ohm-cm, gridline spacing can bewider, for example about 1200 microns rather than 500 microns or less.This would result in less shading of the light incident on the cell,yielding a higher efficiency. It could also allow the use of widercontacts and metal fingers, possibly reducing the cost of thefabrication process. Further, with a lower base resistivity, localizeddoping of the front surface adjacent to electrical contacts can morereadily be reduced or omitted.

Referring to FIG. 3 c in the previous example, the width of trenches 44,and thus the width of wiring 57 shown in FIG. 3 d, was, for example,between about 10 and about 50 microns. With lower base resistivity,these trenches will be farther apart, and may also be wider, for examplebetween about 10 and about 100 microns, in some embodiments betweenabout 30 and about 80 microns, for example 60 or 65 microns.

Thus in various embodiments of the present invention, a photovoltaiccell may be fabricated by starting with a donor wafer having the lowerresistivity just described. Fabrication begins with a lightly dopeddonor wafer of a first conductivity type. A first surface of the donorwafer is doped to a second conductivity type, opposite the first. Thisfirst surface will be the back surface of the photovoltaic cell, and thep−n junction has just been formed by this doping step. One or morespecies of gas ions is implanted through the first surface, defining acleave plane. The first surface of the donor wafer is affixed to areceiver element, with electrical contact formed between them, forexample by an interposed metal layer. A thin semiconductor lamina isformed by cleaving from the donor wafer at the cleave plane as has beendescribed. The second surface of the cell, formed by exfoliation, willbe the light-facing surface of the lamina. This second surface ispassivated by any of the means described, for example by deposition ofan appropriately charged dielectric. Electrical contact is formed to thefront surface, completing the photovoltaic cell.

Summarizing, a photovoltaic cell according to embodiments of the presentinvention may be formed by providing a donor wafer doped to a firstconductivity type, wherein the resistivity of the donor wafer is lessthan about 0.5 ohm-cm; defining a cleave plane within the donor wafer;affixing the first surface of the donor wafer to a receiver element;cleaving a semiconductor lamina from the donor wafer at the cleave planewherein the semiconductor lamina remains affixed to the receiverelement; and completing fabrication of the photovoltaic cell, whereinthe photovoltaic cell comprises the semiconductor lamina.

Formation has been described of a photovoltaic cell comprising asemiconductor lamina which in turn comprises a base region. The baseregion comprises at least a portion of a base of the photovoltaic cell,wherein the resistivity of the base region of the semiconductor laminais less than about 0.5 ohm-cm, and wherein the semiconductor lamina hasa thickness less than about 50 microns. As in other embodiments, thelamina may be significantly thinner, for example 20 microns, 5, microns,2 microns, or less. The resistivity of the base region may be betweenabout 0.005 and 0.5 ohm-cm.

In some embodiments, the p−n junction is at the back of the cell. Thissurface was the first surface, which was bonded to the receiver element.Thus, in these embodiments the semiconductor lamina has a front surfaceand a back surface, wherein incident light enters the semiconductorlamina at the front surface during normal operation of the photovoltaiccell, wherein the semiconductor lamina comprises a p−n or n−p junctionof the photovoltaic cell, and wherein the p−n or n−p junction of thephotovoltaic cell is nearer to the back surface of the lamina than tothe front surface of the lamina. Within about 300 nm of the back surfaceof the lamina, the peak dopant concentration is greater than about1×10¹⁹ atoms/cm³.

As has been described, embodiments of the photovoltaic cell of thepresent invention have no FSF. Thus in these embodiments, the portion ofthe semiconductor lamina within 300 nm of the front surface hassubstantially the same resistivity as the resistivity of the base regionof the semiconductor lamina.

Example: Amorphous FSF

In another embodiment, the surface created by exfoliation is passivatedby amorphous silicon. Turning to FIG. 5, fabrication begins as in priorembodiments, with a lightly doped donor wafer (not shown) of a firstconductivity type, for example n-type. Its first surface 10 is doped,for example by diffusion doping, to a second conductivity type oppositethe first, for example p-type, forming heavily doped p-type region 16,and forming the p−n junction. After implantation of one or more speciesof gas ions, for example hydrogen and/or helium, to define a cleaveplane, first surface 10 is affixed to receiver element 60 with aconductive layer 12 disposed between them. A semiconductor lamina 40 iscleaved from the donor wafer at the cleave plane, creating secondsurface 62.

Next a very thin intrinsic amorphous silicon layer 72 is deposited onsecond surface 12, followed by a heavily doped amorphous silicon layer74 of the first conductivity type, in this example heavily doped n-type.Heavily doped amorphous layer 74 serves as an FSF. This layer can bedeposited at relatively low temperature. Next transparent conductiveoxide (TCO) 110 is formed on heavily doped amorphous layer 74.Appropriate materials for TCO 110 include aluminum-doped zinc oxide, aswell as indium tin oxide, tin oxide, titanium oxide, etc.; this layermay serve as both a top electrode and an antireflective layer. Inalternative embodiments, an additional antireflective layer may beformed on top of TCO 110. Wiring can be formed on TCO 110, and, as inprior embodiments, photovoltaic assembly 84 just fabricated can beaffixed to a substrate or superstrate. Each photovoltaic assemblycomprises a photovoltaic cell, and the photovoltaic cells are preferablyelectrically connected in series, forming a photovoltaic module.

Any of the fabrication methods described herein can be combined, andfurther can be combined with any of the fabrication methods described inSivaram et al., the Hemer et al. application filed on even dateherewith, or any of the other incorporated applications. A variety ofembodiments has been provided for clarity and completeness. Clearly itis impractical to list all embodiments. Other embodiments of theinvention will be apparent to one of ordinary skill in the art wheninformed by the present specification.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A method for making a photovoltaic cell, the method comprising:providing a donor wafer doped to a first conductivity type; doping afirst surface of the donor wafer to a second conductivity type oppositethe first conductivity type; defining a cleave plane within the donorwafer; affixing the first surface of the donor wafer to a receiverelement wherein a reflective metal layer is disposed between thereceiver element and the donor wafer; cleaving a semiconductor laminafrom the donor wafer at the cleave plane wherein the semiconductorlamina remains affixed to the receiver element, and the lamina comprisesa second surface opposite the first surface; and completing fabricationof the photovoltaic cell, wherein the photovoltaic cell comprises thesemiconductor lamina; wherein the first surface comprises an emitterregion of the photovoltaic cell, and wherein the second surface is alight facing surface of the lamina.
 2. The method of claim 1, whereinthe semiconductor lamina has a thickness, measured perpendicular to thereceiver element, less than about 80 microns.
 3. The method of claim 2,wherein the thickness of the semiconductor lamina is between about 0.2and about 5 microns.
 4. The method of claim 1, wherein the donor waferis substantially at least one of monocrystalline, polycrystalline, andmulticrystalline silicon.
 5. The method of claim 1, wherein the step ofdefining the cleave plane comprises implanting one or more species ofgas ions through the first surface.
 6. The method of claim 5, whereinthe one or more species of gas ions comprise at least one of hydrogenand helium ions.
 7. The method of claim 1, wherein the step ofcompleting fabrication of the photovoltaic cell comprises forminglocalized areas of dopant on the second surface, and applying laserenergy to the second surface to form heavily doped regions of thesemiconductor lamina adjacent to the localized areas.